1. Field of the Invention
The present invention relates to a light pipe and a plane light source unit, which are excellent in light utilizing efficiency and uniformity of brightness, and which can constitute a liquid-crystal display device bright and easy to view.
The present application is based on Japanese Patent Application No. Hei. 11-353139, which is incorporated herein by reference.
2. Description of the Related Art
Heretofore there were known a transmission type liquid-crystal display device and a reflection type liquid-crystal display device. A plane light source unit was constituted by a light pipe having a light output means constituted by prismatic structures, and a light source disposed on a side of the light pipe. Such a plane light source unit was used as a back-lighting system unit for a transmission type liquid-crystal cell in the transmission type liquid-crystal display device. On the other hand, the plane light source unit was used as a front-lighting system unit for a reflection type liquid-crystal cell in the reflection type liquid-crystal display device. There was, however, a problem that a dark portion was produced in light emission of the light pipe to thereby cause lowering of display quality.
It is an object of the present invention to develop a light pipe by which it is possible to form a light emission condition which is excellent in uniformity of brightness because when the light pipe is used to form a plane light source unit, partial dark portions as shades are hardly produced. By such a light pipe, it is possible to form a liquid-crystal display device which is bright and easy to view.
According to the present invention, there is provided a light pipe comprising a plate-like member at least including rectangular upper and lower surfaces, and an incidence side surface which is one of side surfaces between the upper and lower surfaces, and a light output means formed in one of the upper and lower surfaces so that light incident on the incidence side surface exits from the other one of the upper and lower surfaces through the light output means, wherein a radius of curvature of a corner portion formed at a point of intersection between the incidence side surface and each of side surfaces adjacent to the incidence side surface is not larger than 0.3 mm. There is further provided a plane light source unit in which a linear light source is disposed on the incidence side surface of the light pipe. There is further provided a transmission type or reflection type or reflection-transmission double type liquid-crystal display device which comprises at least such a plane light source unit and a liquid-crystal cell.
According to the present invention, in the case where a plane light source unit is used, partial dark portions as shades, especially dark portions as shades because of corner portions at ends of an incidence side surface are hardly produced so that a light emission state excellent in uniformity of brightness can be formed. Hence, there can be obtained a light pipe which is excellent in light utilizing efficiency and which can constitute a liquid-crystal display device bright and easy to view.
That is, the inventors of the present invention have eagerly made investigations in order to prevent such dark portions from being produced in the plane light source unit which uses a light pipe having a light output means constituted by slopes such as prismatic structures, etc. In the meanwhile, the inventors have found that the fine shape of the corner portions at ends of the incidence side surface of the light pipe has a strong influence on the production of stripe-like shades in a direction of light transmission at the ends of the incidence side surface. They also have found that the more the corner portions are rounded, the more the quantity of incidence of light or the quantity of backward transmission of light due to reflection or refraction decreases or the more the width of strip-like shades increases. The findings are conceived to be based on the characteristic of the light pipe as follows. In the light pipe having the light output means, incident light is transmitted through the light pipe while prevented as much as possible from being scattered. The transmitted light is made to exit from one of the upper and lower surfaces through reflection, especially through total reflection by the light output means. Hence, if variation in light incident on the incidence side surface occurs, the variation will cause difference between brightness and darkness in output light.
Incidentally, in a light pipe having a scatter type light output means constituted by dot-like uneveness, embossed uneveness, or the like, the aforementioned shade problem hardly occurs even in the case where the roundness of corner portions at ends of the incidence side surface is relatively large. In this case, however, if output light exits at a large angle of about 60 degrees, dark light emission occurs in the frontal (perpendicular) direction, so that frontal luminance runs short. Hence, it is difficult to achieve high-luminance transmission type display when the light pipe is applied to a back-lighting system. Furthermore, it is difficult to put the light pipe in practical use because display light is scattered and can not be viewed when the light pipe is applied to a front-lighting system.
Hence, the aforementioned shade problem is peculiar to a light pipe having a light output means constituted by slopes such as prismatic structures. On the other hand, the light pipe, however, exhibits the aforementioned characteristic. Hence, the light pipe is excellent in directivity of light reflected by the slopes, so that frontal light favorable to viewing in a transmission mode can be formed efficiently. Hence, bright display with high luminance can be obtained. Furthermore, even in the case where the light pipe is applied to a front-lighting system, display light can be efficiently transmitted in the frontal direction so that display bright and easy to view can be obtained.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.